Nickel chromium alloy and electrical resistance heating elements made thereof



Dec. 3, 1957 NICKEL CHROMIUM ALLOY AND ELECTRICAL RESISTANCE w. BETTERIDGE E'rAx. 2,815,283

HEATING ELEMENTS MADE THEREOF Filed July 14, 1953 C /"/v/n 400 Y 300 /l salaam-P55 f5/v7 0ov owa-/a TI'ORNEY NICKEL CHROMIUM. ALLOY, AND ELECTRICAL RESISTANCE HEATING ELEMENTS MADE THEREQF Walter Betteridge, Solihull, audi Harry Lewis, Kings Heath, Birmingham, England, asSignOrs to` The International Nickel Company, Inc., New York, N. Y., a corporation of Delaware Application Juiy i4, 195s, sexismo. 367,852

Claims priority, application. Greafritain` July- 1'1, 1952 2 Claims. (Cl. 75171)- The present invention relates to a special nickel-chromium heat resistant alloy and, more particularly, to electrical resistanceI heating elements made of such alloys and having improved service life when subjected in use to` elevated temperatures, especially under conditions involving repeated heating and cooling.

It is known that nickel-chromium alloys used for electrical resistance heating elements may advantageously contain small amounts of both calcium andlcerium (for other rare earth metal) for the purpose of increasing the, Service life, and that these alloys may contain small amounts of other elements, e. g., silicon. In making the alloys, the cerium is commonly used as Mischmetall, and the term cerium is used herein to mean not only cerium itself but also any other rare earth metals present. In practice the silicon contents of nickel-chromium alloys containing both calcium and cerium have hitherto been very low, e. g., of the order of 0.5%. While such nickel-chromium alloys have been found beneficial when employed as electrical resistance heating elements, demands -by industry for heating elements with greatly improved service lives have placed further burdens on such alloys with the result that the problem of providing improved alloys to moet the needs of industry has been greatly accentuated. Although many attempt-s were made to meet the needs and demands of industry, none, as far as applicants are aware, were entirely successful when carried into practice commercially on an industrial scale.

We have discovered that by employing a higher and critical silicon content in conjunction with a very low and critical cerium content, it is possible very considerably to increase the service life of an electrical resistance element made of such an alloy. To be forgeable, the alloy must -also contain a small and critical amount of calcium.

ltis an object of the present invention to provide a special heat resistant nickel-chromium alloy having improved service life at elevated temperatures.

It is another object of the present invention to provide special electrical resistance heating elements characterized by improved performance at elevated service tempera/tures, especially under conditions involving repeated heating and cooling.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

Figure 1 is a curve showing the critical eiect of silicon on the average life in hours of a nickel-chromium alloy containing a critical amount of cerium and tested in accordance with designation B 76-39 of the American Society for Testing Materials (ASTM); and l,

Figure 2 depicts a curve showing the critical effect of cerium on the average life in hours of a nickel-chromium alloy containing a critical `amount of silicon likewise tes-ted in accordance with the aforementioned ASTM designation B 7 6-39.

According to the invention, nickel-chromium alloys for fnited States Patent Of 2,8%,233 Paitentetiv Dec. 3, 1957 electrical resistance heating; elements contain from 10 to 2.5%, chromiumrfrom 0.0.05 to 0.051%x calcium. 0.01 t0 0.1% cerium and 1.1i5 to 2%. sililcomthe balance (except for impurities) beingfnickel?. Preferably thev elements are present in nai:newer:ranges,l namely from 15 to. 2.5% Chromium, from @im to. 0.03 7o.- calcima, from` 0.025 t0 0.06% cerium and from 1..4;.to. 1.6% silicon.

Although the invention is concerned with; nickel-chromium alloys, as distinguished from nickel-chromiumiron alloys, iron isotten presenta as.- an impurity inthe raw materials and inV consequence the alloys of the invention may contain up.. to 2% iron.. Moreoveri some: of thel nickel (in an amount up to 15% of the total `alloy) may be replaced by: cobalt.

rElie alloys contain various: other elements without detriment, namely up to 1%. aluminum, up. to, 0.3% carbon, up to. 0.16%.v coppeii-l andg up to.y 3%. manganese. The impurities present may: includey tracesv of various elements, e. g., titanium.

ln evaluating heat resistant alloys of the type containing nickel and 20% chromium for use yas electrical resistance heating elements, `an accelerated life. test is employed in accordance. with.- the American Society for Testing Materials designation B 'I6-39. In this test, the alloy-'specimen in they form of la wire measuring about 12 inches long and. having a diameter corresponding to not larger than, No. 20 American Wire Gauge ('A. W. G.) nor smaller than No. 22 A. W. G. (i. e., within the range of about 0.025 inch to 0.032 inch) is subjected to intermittent heating and cooling under prescribed conditions at a temperature of about 2l50 F. (1177 C.), the heating being accomplished by passing electric current through the wire. The service lives of electrical resistance wires of a number of alloys have been measured in accordance with the aforementioned ASTM test. The percentage compositions of some of the alloys tested and the results obtained are given inthe following table:

The iirst alloy (No. l) was a typical alloy as marketed hitherto. The second `alloy (No. 2) had a silicon content which, though much higher than usual hitherto, was still below the preferred range of the present invention. The third alloy (No. 3) has silicon and cerium contents both within the preferred ranges. The results of tests conducted on the alloy provided by the invention indicated that markedly improved service livesare obtained when the alloy contains about 0.03% to 0.05% cerium, particularly when the alloy contains 1.4% to 1.6% silicon.

The way in which the service life varies with the silicon content is illustrated by Figure 1 of the accompanying drawings, which is a graph showing the average lives obtained with electrical resistance wires of alloys containing about 0.010% calcium, 0.04% cerium, 0,2% aluminum, 20% chromium and 0.4% iron and of varying silicon contents. It will be seen that as the silicon content rises above the normal low ligure, there is no appreciable increase in the service life until it is about 1.0%. At this ligure the service life begins to increase at a rate which itself rapidly increases. Between 1.15 and 1.4%, the increase is most striking, the service life rising to at least three times the value at 0.6% silicon. When the silicon content exceeds 1.4%, the lite remains approximately constant up to 2%.

The critical nature of the cerium content is illustrated by Figure 2 of the accompanying drawings, which is a graph showing the average lives Obtained with electrical resistance wires of alloys containing about 0.010% calcium, 0.2% aluminum, 1.5% silicon, 20% chromium and 0.4% iron, and of varying cerium contents. It will be seen that in the narrow range of 0.03 to 0.05% cerium, the best lives are obtained. Generally, alloys provided by the invention exhibit service lives at 2150 F. (1177 C.) as determined by the ASTM designation B 76-39 of the order of about 400 hours and higher, while the best commercially available alloys of the 80-20 nickelchromium type of which we are aware do not show good lives of this order when tested in this manner.

The term service life in this specification refers in all cases to the total life to burn-out of the Wire, since this occurs before the resistance of the wire has increased by 10 percent.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope 0f the invention and appended claims.

We claim:

1. An electric resistance heating element made of a nickel-chromium alloy consisting essentially of 15% t0 25% chromium, 0.01% to 0.03% calcium, 1.4% to 1.6% silicon, 0.03% to 0.05% cerium, up to 15% c0- balt, up to 2% iron, up to 3% manganese, up to 1% aluminum, up to 0.16% copper, up to 0.3% carbon, and the balance consisting essentially of nickel, said electric resistance heating element being characterized by markedly improved service life of at least 400 hours when subjected to intermittent heating and cooling at an elevated temperature of 2150 F. in accordance with ASTM designation B 76-39.

2. A heat resistant nickel-chromium alloy adapted for the manufacture of electrical resistance heating elements and consisting essentially of 15% to 25% chromium, 0.01% to 0.03% calcium, 1.4% to 1.6% silicon, 0.03% to 0.05% cerium, up to 15% cobalt, up to 2% iron, up to 3% manganese, up to 1% aluminum, up to 0.16% copper, up to 0.3% carbon, and the balance consisting essentially of nickel, said alloy being characterized by improved service life of at least 400 hours when subjected as an electrical resistance wire to intermittent heating and cooling at an elevated temperature of 2150 F. in accordance with ASTM designation B 76-39.

References Cited in the file of this patent UNITED STATES PATENTS 1,465,547 Driver Aug. 2l, 1923 2,174,919 Kay Oct. 3, 1939 2,581,420 Lohr Jan. 8, 1952 OTHER REFERENCES Ser. No. 124,858, Hessenbruch (A. P. C.), published June 1, 1943. 

1. AN ELECTRIC RESISTANCE HEATING ELEMENT MADE OF A NICKEL-CHROMIUM ALLOY CONSISTING ESSENTIALLY OF 15% TO 25% CHROMIUM, 0.01% TO 0.03% CALCIUM, 1.4% 1.6% SILICON, 0.03% TO 0.05% CERIUM, UP TO 15% COBALT, UP TO 2% IRON, UP TO 3% MANGANESE, UP TO 1% ALUMINUM, UP TO 0.16% COPPER, UP TO 0.3% CARBON, AND THE BALANCE CONSISTING ESSENTIALLY OF NICKEL, SAID ELECTRIC RESISTANCE HEATING ELEMENT BEING CHARACTERIZED BY MARKEDLY IMPROVED SERVICE LIFE AT LEAST 400 HOURS WHEN SUBJECTED TO INTERMITTED HEATING AND COOLING AT AN ELEVATED TEMPERATURE OF 2150*F. IN ACCORDANCE WITH ASTM DESIGNATION B 76-39. 